A citizens band ( CB ) radio antenna is a twist designed to do two thing : It capturesradio - frequencysignals that are then converted to electrical signals by the receiver , and it takes electrical signal from the transmitter and converts them into radio - frequency signals . This second function is where tuning comes into drama , because an antenna has toradiateradio - frequency signals , something that ’s done better when the length of the antenna precisely matches the wavelength of the transmitted radio absolute frequency .
you’re able to see the right length of an transmitting aerial by using a formula :
Wavelength ( in substructure ) = 984 / frequency ( in megahertz )
The CB portion of the spectrum begins at 25.01 MHz , so a full wavelength feeler would be a bit more than 39.34 invertebrate foot long . That ’s plainly a little long to attach to your bumper , so hoi polloi tend to utilize antenna that are a fraction of the wavelength : 1/2 , 5/8 , 1/4 and 1/8 are all vernacular wavelengths for antennas . In the case of CB , the 1/4 antenna at just under 10 feet long is the uncouth " whip " that you may see on cars and trucks .
The trouble is that there are 40 channels on forward-looking CB transceivers , each agree to a different frequency . It ’s not practical to have a separate antenna for each absolute frequency , so aerial couturier have to compromise , normally pick a relative frequency in the middle of the spread and choose the antenna length to check .
When a compromise like this is made , you have to see whether it ’s a good via media . This is done by measuring the Standing Wave Ratio ( SWR ) of the antenna and cable between the antenna and tune the antenna until the SWR is satisfactory .
Every transmitting aerial and every antenna feed - line have a characteristicimpedance , or opponent to electrical current . In an ideal situation , the impedances of line and antenna match dead , and 100 percent of the electrical energy send to the feeler is convert to radio energy and glow into the air . In a less than idealistic lawsuit , when the impedances are n’t utterly matched , some of the electric vigour mail to the antenna wo n’t be change to radio energy , but will be reflected back down the provender - bank line . The vim muse back from the antenna causesstanding wavesof electric Energy Department in the feed - line . ( An model of abide waves outside the electronics worldly concern is found in river rapid . When water passes around and between boulders it may form a wafture that does n’t go up or down the river , it just stays in one place . That is a standing moving ridge of water system . ) The ratio of highest voltage on the line to low-down is the brook undulation proportion . In the dead play off system , the SWR is 1:1 .
To tune up an antenna , use an SWR metre attached between the transmitter and antenna provender - line . Depending on the time , you may either apply a button on the measure to generate a signaling on the various channels , or distinguish the mike on the CB transceiver to return a signaling while you look at the SWR reading . In general , if the SWR never goes above 1.5:1 , you ’re in ripe shape . If the SWR does go above 1.5:1 , then watch out the meter on unlike frequency to see the trend grow : The SWR will be greater either on the higher channels or the lower . If the SWR is greater on the broken channels , then try gradually lengthen the antenna by moving it in the al-Qaida . If the SWR is greater on the higher channels , try shortening the aerial .
Do be aware that the electrical ground of the antenna , the structure around the antenna and any other antennas near the CB antenna can all impact the antenna ’s impedance and the SWR . There are enough variables that tune an aerial blend art and scientific discipline , but your equipment and tuner contacts will all be thankful if you take the time to tune up .
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